This application is based on application No. 11-325467 filed in Japan, the content of which is hereby incorporated by reference.
(1) Field of the Invention
The present invention relates to a flash device used for an image forming device, a camera, and the like, and more specifically to a technique to supply power to the flash lamp.
(2) Description of Related Art
Some electrophotographic image forming devices, such as a laser printer, form a visible image from an electrostatic latent image by using toner, transfers the toner onto a recording medium such as a sheet of paper, and has a flash lamp in a fixing device flash to fix the transferred toner onto the sheet of paper. As another example of a flash device use, a flash device is used for a camera, and has a flash lamp flash to irradiate a subject. An example of such a flash device is disclosed by Japanese Laid-Open Patent Application No. 60-128475.
FIG. 1 is a circuit diagram showing a construction of this flash device. As shown in the figure, the flash device comprises the following elements: a diode bridge rectifier BR (hereafter just called xe2x80x9cbridge rectifierxe2x80x9d), which rectifies an AC (alternating current) voltage of 200 volts outputted from a commercial AC power supply AC; a flash lamp FL that is a discharge tube into which xenon gas is filled; diodes D1xcx9cDn which prevent a backward current; discharge capacitors C1xcx9cCn; and switches SW1xcx9cSWnxe2x88x921. Under control of a flash power supply control circuit (not shown in the figure), these switches SW1xcx9cSWnxe2x88x921 are switched to selectively establish a first connection state (shown by solid lines in the figure) and a second connection state (shown by dotted lines).
When the capacitors C1xcx9cCn should be charged, the switches SW1xcx9cSWnxe2x88x921 are switched to establish the first connection state, where the capacitors C1xcx9cCn are connected in parallel and the bridge rectifier BR is connected to negative terminals of the capacitors C1xcx9cCn. As a result, the capacitors C1xcx9cCn are each charged up to 280 volts, which are equal to a peak output voltage of the bridge rectifier BR.
After the capacitors C1xcx9cCn have been charged in this way, the switches SW1xcx9cSWnxe2x88x921 are switched to establish the second connection state to have the flash lamp FL emit light. As a result, negative terminals and positive terminals of the capacitors C1xcx9cCn are connected, and the capacitors C1xcx9cCn are connected in series with the flash lamp FL. This boosts a voltage applied to between main electrodes of the flash lamp FL to an n-fold voltage of one capacitor. When a trigger signal is applied to a trigger electrode of the flash lamp FL to which the boosted voltage is impressed, xenon gas inside the flash lamp FL is excited, and electrical resistance of the flash lamp FL reduces. As a result, electrostatic energy is supplied from the capacitors C1xcx9cCn to the flash lamp FL, so that a discharge current flows between the main electrodes, and the flash lamp FL emits light.
With a conventional flash device like the above, when the output voltage of the bridge rectifier BR is lowered, each capacitor can have a low withstand voltage, so that the cost of parts in the flash device can be reduced.
However, with the conventional flash device, the capacitor C1 continues to be charged due to an output from the bridge rectifier BR while the flash lamp FL emits light. As a result, voltage continues to be supplied to the flash lamp FL, so that the flash lamp FL unnecessarily continues to emit the light. In addition, the flash lamp FL continues to emit the light even after the connection state has been switched back to the first connection state because power continues to be supplied via the bridge rectifier BR to the flash lamp FL. In this way, a conventional flash device has a drawback in that it cannot stop the flash lamp FL from emitting light with a desired timing and control a quantity of light emitted by the flash lamp FL.
The present invention is made in view of the above problems, and aims to provide a flash device that can reliably control illumination of a flash lamp.
The above object can be achieved by a flash device which includes: a DC power supply; a flash lamp; a charging path; a discharging path; a first group of charge/discharge elements that are charged by the DC power supply, and discharge electric charge to the flash lamp; a first switch that selectively establishes (a) a parallel state in which the first group of charge/discharge elements are connected in parallel with one another, and connected to the DC power supply via the charging path and (b) a series state in which the first group of charge/discharge elements are connected in series with the flash lamp via the discharging path, the first group of charge/discharge elements being connected in series with one another; a second switch that is positioned in the charging path, and selectively connects and disconnects the first group of charge/discharge elements to and from the DC power supply; and a controller that establishes a first control state to have the first group of charge/discharge elements charged, and a second control state to have the flash lamp emit light. In the first control state, the second switch performs connection and the first switch establishes the parallel state. In the second control state, the second switch performs disconnection and the first switch establishes the series state.
Unlike with a conventional flash device, capacitors in the above flash device are not charged during illumination by the flash device. This reliably prevents the present flash lamp from continuing to emit light unnecessarily, and therefore an amount of the light can be suitably controlled.